How do I rotate a bone without effecting the connected bones?
Go into Bone Edit Mode of the Armature. Tab
Select the spiked_boulder bone and duplicate it. ShiftD
For further convenience, let's scale this bone down, so we can discern it later on. Let's name it spiked_boulder_rotation.
Parent spiked_boulder_rotation to spiked_boulder. CtrlP
Assign/Parent the spiked boulder mesh to the spiked_boulder_rotation bone.


Now you can animate the movement of the spine in a standard fashion. The child spiked_boulder_rotation will rotate along its parent moving the geometry. Now animated the Y Rotation of the spiked_boulder_rotation if you want to additionally rotate the spike geo.